Lignin is a highly abundant biopolymeric material (second only to cellulose) and can be derived from wood via processes that have been used for many years. Lignin is an amorphous, highly branched polyphenolic macromolecule with a complex structure, and the material typically forms about ⅓ of the dry mass of woody materials. The general structure of native lignin is shown in FIG. 1; however, lignin structure is known to be significantly altered when lignocellulosic material (e.g., wood or other plant materials) are treated under conditions intended to separate the lignin from the cellulose.
Lignin provides structure to woody materials and is the component responsible for the strength of wood against mechanical stress. The physical and chemical properties of lignin can vary depending upon the wood species, the botanical origin, and the region from which the wood is harvested, and the process by which the lignin is isolated. Lignin typically is obtained from pulping processes such as used in the paper and biorefinery industries where the lignin is separated from cellulosic fibers. Large quantities of modified lignin are made available yearly from pulping processes as well as bio-ethanol digestion and saccharification processes. For example, the global production of isolated lignin from sulphite processes is about 1 million tons/year, and Kraft processes provide around 100,000 tons/year of lignin. The “technical lignin” arising from such methods has undergone severe hydrolytic degradation imparting a highly reactive structure and a relatively low molecular weight. This can render technical lignins unsuitable for many value added applications despite its aromatic and somewhat polymeric nature.
Previously, the majority of produced technical lignin (˜90%) has been used as a combustion fuel to provide energy for heat or power production. Lignin also has been used as an additive in various low volume and niche applications, such as being used as a dispersant, in concrete admixtures, as a binder in mining operations (e.g., copper, carbon black, and coal), and as an adhesive. Efforts to use lignin as a source of valuable carbon fiber have also been made with limited success. The common feature for such previous commercial uses of lignin is that, in all cases, the lignin only serves as an additive to produce relatively low added value products.
Most efforts to utilize lignin previously have been limited by various factors that impart in lignin characteristics that define it as an unreliable precursor to polymer production. This is because lignin (and more specifically technical lignin) offers relatively unpredictable polymerization characteristics, depending upon its source, the pulping (or other process) from which the lignin arises, and the degree of delignification to which the plant materials were subjected. More specifically, the highly functional character of lignin (i.e., rich in phenolic and aliphatic OH groups, as well as reactive benzylic carbons) induces a variety of potential polymerization sites and heat instability in such materials. Both factors promote gelation processes under polymerization conditions or when the temperature increases close to and/or above the glass transition temperature (Tg). Heating of lignin at an elevated temperature converts it to a condensed from and makes it rigid and less reactive. The irreversible formation of such gels precludes lignin from becoming and being considered as an integral part of modern synthetic polymer and composite production lines. In addition, the relatively low molecular weight (a few thousands) for lignin derived from commercial pulping and biorefinery operations makes lignin unsuitable for higher end applications, such, for example, high performance, heat stable engineering thermoplastic applications.
Chemically, lignin has a variety of functional groups, namely hydroxyl, methoxyl, carbonyl and carboxyl groups. Phenolic hydroxyl groups in the aromatic rings are the most reactive functional groups in the lignin and can significantly affect the chemical reactivity of the material. Higher end uses of lignin have not previously been achieved because of its structural complexity, augmented reactivity, and thermal instability. To improve upon this limitation, different types of modifications have been proposed with objectives to increase its chemical reactivity, reduce the brittleness of lignin-derived polymers, increase its solubility in organic solvents, and improve the ease of processing the lignin. For example lignin modification with propylene oxide for preparation of engineering plastic and polyurethane foam has been proposed. This type of modification results in the formation of lignin polyol derivatives, which in turn improves the solubility and uniformity of the lignin. During the modification, the majority of phenolic hydroxyl groups are converted to aliphatic hydroxyl units. Thus, more reactive hydroxyl groups become readily available. Previous methods have consisted of mixing the solid lignin into pure propylene oxide in the presence of a base (usually KOH) at a temperature of 150-200° C. for 1 to 2 hours.
In general, lignin is a green biomaterial which has a great market potential in renewable energy and biopolymeric materials. Despite the previous attempts in the field, new methods and materials utilizing lignin are desirable.